The present invention relates to laser wavelength tuning, and more particularly to thermally tuning laser wavelength output.
Tunable lasers are highly desirable for multi-wavelength telecommunication links. Though there are a variety of laser structures that can tune the output wavelength, such structures generally suffer from various disadvantages such as low output power, small tuning range, poor spectral quality, or short lifetime. Compared to standard fixed wavelength distributed feedback (DFB) lasers, tunable lasers almost always compromise some functionality to obtain wavelength tunability.
An alternative technique to using a single tunable laser is to select a particular laser from a multi-wavelength array. A structure for performing such a technique may include an array of multiple wavelength lasers fabricated on one chip. Some means, for example a movable mirror, couples a particular laser of the array, to an output fiber. Only one laser is operated at a time, with the choice of lasers governed by the desired wavelength.
To keep the number of lasers manageable, the wavelength spacing between the lasers is made fairly large, and the temperature of the chip is adjusted by changing the temperature. Since high performance semiconductor lasers are usually stabilized with a thermo-electric (TE) cooler, the temperature can be easily adjusted electronically using a TE cooler. The tuning rate of DFB lasers in the 1.55 um communication band is about 0.1 nm/C. Thus changing the temperature from about 10xc2x0 C. to 40xc2x0 C. can allow 3 nm tuning without deleterious effects on the device performance or lifetime.
Unfortunately, when a TE cooler is used to fine tune the wavelength, the tuning time is relatively long, on the order of a second. For many applications, such as wavelength provisioning in the SONET telecommunication format, much faster tuning times on the order of milliseconds is preferred.
The present invention describes a number of techniques of applying localized heating in a multi-wavelength semiconductor laser array for rapidly fine tuning the output wavelength. Current can be driven along a laser stripe, and thus obviating the need for a separate heater. One additional contact is used per laser for thermal tuning. Alternatively, lateral metallization can be applied adjacent to the lasing stripes, with all the lateral metallization electrically connected to one contact. Applying current through this contact to the active stripe causes the active element to heat. In this technique, one extra electric contact is used for the entire array. Another method is to apply a reverse bias to at least one adjacent laser of the array to cause current to flow in the top doped cladding, which has a negligible effect on the optical power output, but heats the laser element. In a closely spaced array, thermal cross talk between the lasers can also be used to tune the wavelength. In such a design, only the selected laser is optically coupled to the output, thus activating adjacent lasers does not affect the optical output directly. However the heat generated by the adjacent laser will thermally tune the optically coupled device. If optical cross-talk is an issue, the adjacent laser can also provide heat without light if the adjacent laser is reverse biased, particularly strongly reversed biased. Individual thin film heaters can also be used in the array. In a set of operating conditions, the individual film heaters, or other heaters discussed herein, rapidly tune the wavelength of the laser while the TE cooler has time to respond. Thus, the heater would only be used for a short time during the tuning cycle, and little extra power would be required otherwise.
In one embodiment, the present invention comprises a thermally tuned optical device. The optical device comprises a diode laser having a substrate, a waveguide, and an active region between the substrate and the waveguide. The optical device further includes an electrical contact on the substrate, the substrate being at a substrate potential, a metal layer in thermal contact with the waveguide, and a first electrical contact and a second electrical contact. The first electrical contact is on the metal layer whereby application of a first potential to the first electrical contact causes the diode layer to lase. The second electrical contact is on the metal layer, whereby application of a second potential to the second electrical contact causes a current to flow between the first electrical contact and the second electrical contact, thereby heating the laser.
In a further embodiment, the present invention comprises a thermally tuned laser array. The array comprises an array of ridge waveguide diode lasers. The ridges are separated by an interstripe area. A metal contact is on top of each ridge, each metal contact therefore corresponding to a laser in the array of lasers. An interstripe metallization is in each interstripe area. One of the metal contacts is set to a potential at least sufficient to cause the corresponding laser to emit light. At least one interstripe metallization in an interstripe area about the one of the metal contacts set to the potential is below that otherwise caused by setting the metal contact to the potential.
In a further embodiment, the present invention comprises a method of thermally tuning a diode laser. In one embodiment the method comprises selecting a laser of the array of lasers. The method further comprises applying a thermal signal to a contact on the array of lasers, and applying a signal to a thermal electric cooler.
These and other aspects of the present invention are more readily understood upon viewing the figures indicated below in conjunction with the following detailed description.